Disposable absorbent products (e.g., disposable diapers) typically include stretchable materials, such as elastic strands, in the waist region and the cuff regions to provide a snug fit and a good seal to a wearer's body. Pant-type absorbent articles further include stretchable materials in the side portions for easy application and removal of the article and for sustained fit of the article. Stretchable materials have also been used in the ear portions of taped diapers for adjustable fit of the article. Useful stretchable materials include elastomeric films, nonwovens, strands of natural or synthetic rubber, elastic scrim, and the like. Typically, these stretch regions are made separately and attached to the diaper using adhesives. In most cases, these designs deliver uniform and unidirectional stretch. Similarly, garments, both disposable and durable, also use elastic elements for improved fit and comfort.
An alternate approach that is capable of delivering multidirectional, non-uniform stretch has been disclosed in copending U.S. patent application Ser. Nos. 10/288,095, 10/288,126 and 10/429,433. This approach involves hot melt printing of one or more thermoplastic elastomers onto a substrate, followed by incremental stretching of the elasticized substrate that then confers the stretch properties of the elastomer to the substrate in a somewhat magnified form. Suitable printing processes disclosed therein include direct gravure, offset gravure, and flexographic printing. Each of these printing methods allows deposition of a predetermined amount of an elastomer in any shape and direction, thus giving a wide variety of design flexibility which ultimately results in improved fit of the overall diaper product. However, improvements are still needed.
Desirably, transfer of such an elastomeric composition from the means used to apply the elastomer to a substrate is substantially complete. Otherwise, the elastomeric composition can oxidize with resulting color degradation, or suffer degradation of other properties. Such degradation, as may result from incomplete transfer is more likely if the application means is heated.
Without being limited by theory, it is therefore important to understand the mechanism of transfer of an elastomer from an application means to a substrate. During this transfer, three forces are relevant. These forces include: i) the adhesive force between the surface of the application means and the elastomer; ii) the cohesive strength of the elastomer (i.e., the resistance of a single portion of an elastomeric composition to separation into two smaller portions); and iii) the adhesive force between the elastomer and the substrate and/or the strength of the substrate. In order to successfully transfer an elastomer to a substrate either one or both of the cohesive strength of the elastomer or the adhesive force between the elastomer and the surface of the application means must be less than the adhesive force between the elastomer and the substrate and/or the strength of the substrate. Typically, this problem has been solved by the use of heated printing processes where the cohesive strength of the heated elastomer is at a sufficiently low value because the elastomer has been maintained in a liquid or semi-liquid state. Thus, transfer of an elastomeric composition from an application means to a substrate typically is achieved through cohesive failure of the elastomer at the point of transfer from the application means to the substrate and a portion of the elastomer remains on the surface of the application means. The above conditions generally apply during, for example, gravure printing of elastomeric adhesives, where the viscosity is relatively low and the adhesive has strong affinity for the walls of the gravure elements and also the substrate. Importantly, cohesive failure means that there is a residual portion of adhesive on the application means that is not transferred.
On the other hand, elastomeric compositions that have good elasticity generally have a higher viscosity at a given temperature than a typical elastomeric adhesive. For reference, typical thermoplastic elastomers used in diapers have viscosities in excess of 1000 Pa·S at 175° C. Increased viscosity translates into a higher cohesive force of the elastomer and a need to heat to a higher application temperature to insure cohesive failure. Such a dynamic poses a problem for conventional direct gravure printing of high viscosity materials, since a point is reached when the cohesive strength of the elastomer either exceeds its adhesive strength with the substrate or it exceeds the strength of the substrate. Such conditions, in turn, result in either a failure of the elastomer to bond to the substrate or damage to the substrate. On the other hand, if temperature is increased to lower cohesive strength, the application temperature of the elastomeric composition may exceed the melting point of the substrate with resulting substrate damage or thermal degradation of the elastomer. Thus, there is a need for an application process that is capable of depositing high viscosity elastomeric compositions on substrates, without damaging these substrates.